Photosensitive polyaldehydes and use in photoimaging

ABSTRACT

Photosensitive polyaldehydes are provided which have the formula: ##STR1## WHEREIN R is a photosensitive end group selected from ##STR2## R 1  is H or n-alkyl of 1-5 carbon atoms, preferably H, R 2  is (a) n-alkanoyl of 1-4 carbon atoms or (b) n-alkanoyl of 1-4 carbon atoms or ##STR3## when R 1  is H, and n is a positive integer of about 10-4000. The polyaldehydes are prepared by the anionic polymerization of the appropriate aldehyde with an initiating amount of an alkali metal or tetraalkylammonium alkoxide of RO in the above formula. 
     The polyaldehydes are useful in articles and methods of relief imaging and in lithographic plates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polyaldehydes, particularly polyoxymethylenepolymers, containing photochemically degradable end groups and toarticles and methods of relief imaging using the same.

2. Relation to the Prior Art

In U.S. Pat. No. 3,991,033 issued Nov. 9, 1976, to Sam, randomcopolymers containing polyoxymethylene chains with interspersedphotosensitive units, werein both ends are joined to oxygen, aredescribed. The units have the formula ##STR4## wherein R¹ and R² are Hor phenyl optionally substituted with up to 5 substituents of loweralkyl, preferably methyl, or --NO₂, n is 0 or 1, R³ and R⁴ are H orlower alkyl, preferably H or methyl, with the provisos

(I) THAT AT LEAST ONE OF R¹ and R² is a phenyl group having at least oneorthonitro substituent,

(II) THAT WHEN N IS 1, R¹ and R² are o-nitrophenyl or H ando-nitrophenyl, said photosensitive linking units being present in anamount of from 0.001 to 0.05, preferably 0.002 to 0.01 unit per --CH₂O-- unit, said polymers having a number average molecular weight of from1000 to 100,000, are preferably having an inherent viscosity of 0.7 to1.5 measured in hexafluoroisopropanol (HFIP) in 0.5% concentration at30° C, which corresponds to a number average molecular weight forpolymers of this type of about 15,000 to about 40,000.

The polymers of that invention are made by intercalation of preformedpolyoxymethylene polymers with substituted dioxolanes, where n = 0, andwith substituted dioxanes, where n = 1, of the formulae: ##STR5## Inthese formulae R¹, R², R³ and R⁴ are as defined above. Dioxolanes aremore readily obtained and are preferred.

Although the polymers of that invention may contain a photosensitiveunit at the end of a polymer chain, they also contain photosensitiveunits randomly interspersed within the polymer chains because of theirmethod of preparation by intercalation. In contrast with the prior artpolymers, the polymers of the present invention contain photosensitiveunits only at the ends of the polymer chains because of their method ofpreparation.

Barzynski, et. al. U.S. Pat. Nos. 3,849,137 and 3,926,636, discloselight-curable compositions having a photosensitive polymer working layercontaining o-nitrocarbinol ester groups.

Limburg, et. al. U.S. Pat. Nos. 3,915,704, 3,915,706, and 3,917,483,disclose imaging compositions and methods based upon photoinduceddegradation of a polyaldehyde catalyzed by an acid, generatedphotochemically from a potential or latent acid. The polyaldehydes donot contain photosensitive units.

Marsh, U.S. Pat. No. 3,923,514, discloses a process for the preparationof relief printing masters which uses a degradable polyaldehyde orpolyketone and a photooxidant which is capable of abstracting anelectron from an oxygen atom of the polymer. The polyaldehydes andpolyketones do not contain photosensitive units.

SUMMARY OF THE INVENTION

According to the present invention there is provided a photosensitivepolyaldehyde having the formula ##STR6## wherein R is a photosensitiveend group selected from ##STR7## R¹ is H or n-alkyl of 1-5 carbon atoms,R² is (a) n-alkanoyl of 1-4 carbon atoms or (b) n-alkanoyl of 1-4 carbonatoms or ##STR8## R¹ is H, and n is a positive integer of about 10-4000.

There is also provided an imagable article comprising a substrate havingon its surface a film of a composition comprising a blend of theaforesaid photosensitive polyaldehyde with a polymer capable of beingcross-linked with aldehyde thermally released from the polyaldehyde.

Also provided is a process for the formation of images comprising: (1)imagewise exposing the aforesaid article to radiation of wavelength inthe range of about 2000A-8000A for a time sufficient to effectsubstantial rearrangement of the photosensitive end groups of thepolyaldehyde; (2) heating the exposed article at a temperature and for atime sufficient to effect cleavage of the rearranged end groups, torelease aldehyde monomer from the polyaldehyde, and to effectcrosslinking of the polymer with the released aldehyde and (3) removingnon-crosslinked polymer from the areas not exposed to radiation.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides polyaldehyde polymers containingphotosensitive, but thermally stabilizing, end groups. It is well-knownthat polyaldehydes degrade thermally and under the influence of base bychain unzippering to give the parent aldehyde. Stabilization of suchpolymers can be achieved by end-capping free hydroxyl groups with etheror ester groups which prevent the initiation of chain unzippering.

The polyaldehydes of the invention have the formula: ##STR9## wherein Ris a photosensitive end group selected from R¹ is H or n-alkyl of 1-5carbon atoms, preferably H,

R² is (a) n-alkanoyl of 1-4 carbon atoms, (preferably acetyl) or (b)n-alkanoyl of 1-4 carbon atoms (preferably acetyl) or ##STR10## when R¹is H, and n is a positive integer of about 10-4000, preferably about15-300.

The polymers of the present invention contain a high proportion ofphotosensitive end groups which confer thermal stability and they are ofthe structural type A or B: ##STR11##

The mechanism of the photolytic and thermal degradation of polymers Aand B is not known for certain, but it is believed to occur as depictedin equations (1) and (2) respectively. ##STR12##

Since the photo-labile groups are located only on the ends of thepolymer chains, essentially complete thermal degradation of thepolyaldehydes can occur once the unzippering process begins. This is incontrast with the polymers of the aforesaid U.S. Pat. No. 3,991,033,which contain photolabile groups intercalated within the polyaldehydechain as well as at the chain ends. The mechanism of the photolytic andthermal degradation of these polymers is believed to occur as depictedin equation (3). ##STR13##

Residual unzippered fragments such as 3a may remain when chain-end E isa non-labile fragment such as acetate. Hence photothermal degradation ofthe polymers may be incomplete and relatively uncontrolled since theintercalation process incorporates photolabile groups randomly withinthe polyaldehyde chains.

The polymers of the present invention are preferably prepared by anionicpolymerization of a suitable aldehyde using an alkali metal salt of thephotolabile end group as initiator. Any straight chain aldehyde(alkanal) containing up to six carbon atoms can be employed, i.e.,formaldehyde, acetaldehyde, propionaldehyde, n-butyraldehyde,n-valeraldehyde and n-hexaldehyde. Formaldehyde is preferred, because ofits ease of anionic polymerization, the stability of its polymers, andbecause the photo-thermally released monomer reacts rapidly with thecross-linkable polymer component in photoimaging applications.

Anionic polymerization of aldehydes is known as discussed by O. Vogl inhis book "Polyaldehydes," Edward Arnold Ltd., London, 1967, pp. 52-66.Relatively strong bases are required to initiate polymerization, forexample, alkali metal alkoxides, soluble hydrides, organometal compoundssuch as butyllithium, and Grignard compounds. When a photolabile groupis employed as an initiator, it is incorporated as an end group into thepolymer chain. It is preferred to use photolabile alkoxides aspolymerization initiators, and the photolabile group is incorporated asan ether end group. Suitable alkoxide initiator counter ions include thealkali metal cations, (lithium, sodium and potassium) as well astetraalkylammonium wherein each alkyl group, alike or different, is from1-6 carbon atoms, such as tetrabutylammonium. It is preferred to employabout 0.00025-0.1 mole of alkoxide initiator per mole of aldehydecharged to obtain polyaldehydes with the desired degree ofpolymerization. To improve initiator solubility in certain hydrocarbonsolvents, it is desirable to add a tertiary amine complexing agent tocomplex with the alkali metal cation. Preferred as a complexing agent isN,N,N'N'-tetramethylethylenediamine.

Solvents inert to the polymerization may be usefully employed. Suchsolvents include aromatic and aliphatic hydrocarbons, ethers, andhindered tertiary amines. Mixtures of solvents may also be usefullyemployed. For polymerization of C₂ -C₆ aldehydes, relatively non-polarhydrocarbon solvents are preferred, i.e., those with a low dielectricconstant, since higher yields of polymer are obtained. Forpolymerization of formaldehyde, the preferred solvents includetetrahydrofuran, cyclohexane, and hexane.

Aldehyde polymerization can be carried out within the temperature rangeof about -80° to about 100° C depending upon the specific aldehydeemployed, reaction solvent, desired degree of conversion to polymer,reaction pressure, etc. With C₂ -C₆ carbon aldehydes, lower temperaturesare usually employed, preferably below -50° C. With formaldehyde,temperatures as high as about 100° C can be used, but preferably about-50° to about 30° C.

Aldehyde polymerization rates can be very fast, and polymerization isusually complete within about one hour. When formaldehyde is employed asthe monomer, it is preferred to prepare anhydrous formaldehyde fromparaformaldehyde in a separate generator, and pass the gaseousformaldehyde into a solution of the initiator. Usually an excess of thecarbinol corresponding to the initiator, i.e., n-nitrobenzyl alcohol orbenzoin, is added as a chain transfer agent during polymerization. Thisinsures the incorporation of a photolabile end group into the majorityof the polymer chains. It is preferred to employ at least about a 100%excess of the carbinol. The polymerization is terminated by addtion of aproton source such as a carboxylic acid. Acetic acid is preferred as theproton source.

The intermediate polyaldehyde, which initially contains hydroxyl endgroups, is encapped by conventional methods, such as using an anhydrideof a carboxylic acid of 1-4 carbon atoms, preferably acetic anhydride,to provide polymer chains with terminal acetate end groups.

The polymers of the invention when R' is H can also be prepared byendcapping the hydroxyl end groups of a polyaldehyde withtris(o-nitrobenzyl) orthoformate to attach o-nitrobenzyl end groups tothe polymer chains. This endcapping reaction is preferably carried outwith an acidic catalyst, preferably boron trifluoride etherate, inheptane solvent at a temperature of 70°-100° C for a reaction time of0.5-3 hours.

The polyaldehydes of the invention normally have a degree ofpolymerization, n, of about 10 to 4000, with 15 to 300 being preferred.The degree of polymerization is determined from melting point dataobtained by differential thermal analysis (DTA). Melting point dataobtained are compared with published data correlating the melting pointwith the degree of polymerization of polyoxymetylene diacetates andpolyoxymethylene dimethyl ethers. Such reference data can be found, forexample, in Tables 61 and 63, pages 164 and 168, of J. F. Walker's book,"Formaldehyde," 3rd edition, Reinhold Publishing Corporation, New York,1964. Since the polyaldehydes of the invention have one ether and oneester end group, an average of the data in Tables 61 and 63 of thereference are used to determine the value of n.

The number of polyaldehyde chains capped with photolabile groups can beconveniently determined by quantitative infrared analysis. A comparisonof the nitro group absorbtivity of the polyaldehyde at 6.5 μ with thecarbonyl stretching frequency (5.75 μ) of the acetate end group usingo-nitrobenzyl acetate as a reference compound gives the percent ofpolymer chains capped with an o-nitrobenzyl group. When the anion ofbenzoin is used as a photolabile initiator, infrared analysis of thepolyaldehyde shows the presence of both acetate and benzoyl carbonylstretching bands, the former at 5.78μ and the latter at 5.9μ. For anequimolar concentration, the ratio of acetate to benzoyl carbonylstretching bands is 1.458.

The imaging process of this inventon employs a suitable imagable articlecomprising a substrate having a radiation sensitive coating on onesurface thereof. This article is formed by coating or impregnating asuitable substrate with a mixture of the photosensitive polyaldehyde ofthe invention and a polymer capable of being crosslinked with thermallyreleased aldehyde following known techniques. By "substrate" is meantany natural or synthetic support which is capable of existing in film ofsheet form and can be flexible or rigid. For example, the substratecould be a metal sheet or foil, a sheet or film of synthetic organicresin, cellulose paper, fiberboard, and the like, or a composite of twoor more of these materials. Specific substrates include alumina-blastedaluminum, alumina-blasted polyester film, silicon wafers, polyesterfilm, polyvinyl alcohol-coated paper, crosslinked polyester-coatedpaper, nylon, glass, heavy paper such as lithographic paper, and thelike.

Preferred as crosslinkable polymers are polyamides because of their easeof reaction with the photothermally-released aldehyde to give tightlycrosslinked polymers. However, other such polymers are the polyureas,polyurethanes, polyamines and the like. The amount of photosensitivepolymer employed is not critical, but sufficient aldehyde should bepresent to effect crosslinking of the polyamide. Weight ratios of about5-30% of polyaldehyde in the polyaldehyde/polyamide mixture aregenerally satisfactory.

When the polyaldehyde/polyamide compositions are coated on metalsurfaces, they are useful for making lithographic printing plates. Forexample, use of a grained aluminum base in combination with aphotolabile polymer mixture results in a developed lithographic plate.The plate, after radiation and image development, is first coated withan aqueous solution of Age (Pitman Co.) and is then contacted with aroller which wets only the photopolymer image with ink. The inked platecan then be used in lithographic printing in the usual way.

The photodegradable polyaldehyde/polyamide compositions may optionallycontain other materials inert to the photodepolymerization reaction.Such materials include thermoplastic and nonthermoplastic binders usefulfor varying the physical properties of the resultant polymeric images.In addition, plasticizers may be added to lower the glass transitiontemperature and facilitate selective stripping. If desired, the polymersmay also contain immiscible polymeric or nonpolymeric organic orinorganic fillers or reinforcing agents which are essentiallytransparent, e.g., the organophilic silicas, bentonites, silica,powdered glass, colloidal carbon, as well as various types of dyes andpigments. Other useful additives which may be employed includesensitizers to improve the efficiency of the radiation and, adhesionpromoters.

The polyaldehyde/polyamide composition can be applied to the substrateas a solution or a dispersion in a carrier solvent, which may besprayed, brushed, applied by a roller or an imersion coater, flowed overthe surface, picked up by immersion or applied to the substrate by othermeans. The solvent is then allowed to evaporate. Useful solvents includethose known in the art to dissolve polyaldehydes and polyamides, e.g.,hexafluoroisopropanol, phenol and substituted phenols including thehalophenols, nitrophenols and cresols, benzyl alcohol and otherfluorinated alcohols such as α,α-di(trifluoromethyl)benzyl alcohol.Useful dispersion solvents include those known to dissolve polyamidesincluding aliphatic alcohols such as butanol and methanol. Coatingtemperatures range from about 0-180° C depending upon the solventemployed. Alternatively, substrate coating may be achieved byhot-pressing a film of the polymers to the substrate or by melt-coatingtechniques. The coating thickness is not critical, but will generally beabout 0.01-25 mils (0.000254-0.635 mm).

The photolabile polymer composition is exposed to radiation ofwavelength in the 2000-8000A range, preferably 2500-5000A. suitablesources of such radiation, in addition to sunlight, include carbon arcs,mercury-vapor arcs, fluorescent lamp with ultraviolet radiation-emittingphosphors, electronic flash units, and photographic flood lamps.

When artificial radiation sources are used, the distance between thephotosensitive layer and the radiation source may be varied according tothe radiation sensitivity of the polyaldehyde. Customarily,mercury-vapor arcs are used at a distance of 1.5 to 24 inches from thephotosensitive layer.

Imagewise exposure, for example, in preparing printing plates, isconveniently carried out by exposing a layer of the photolabile polymercomposition to radiation through a process transparency; that is, animage-bearing transparency consisting solely of areas substantiallyopaque and substantially transparent to the radiation being used wherethe opaque areas are substantially of the same optical density, forexample, a so-called line or halftone negative or positive. Variabledepth images may also be obtained by exposure through a continuous tonetransparency. Process transparencies may be constructed of any suitablematerials including cellulose acetate film and oriented polyester film.

The length of time for which the compositions are exposed to radiationmay vary upwards from a few seconds. Exposure times will vary, in art,according to the nature of the polymer and the concentration and type ofphotolabile end group present, and the type of radiation. However theexpsoure time must be sufficient to effect substantial rearrangement ofthe photosensitive end groups of the polyaldehyde.

Image development is accomplished by depolymerization of the unstablepolymer chains formed in the irradiated areas of the polymer compositionfollowed by crosslinking of the polyamide resin with the thermallyreleased aldehyde. The depolymerization and crosslinking reactions arecarried out at temperatures of about 130°-185° C, preferably about 150°to 160° C, for times of a few minutes to greater than 1 hour. Theunexposed portions of the photopolymer compositions are preferablyremoved by solvent washout using such solvents as hot methanol, ethanol,or cold alcohol/chloroform mixtures. However, in general, the solventused for coating can also be used for washout. After development, thereresults a negative image, i.e., polymer remains under the transparentareas of the process transparency, that is, the areas struck byradiation passing through the transparency.

EMBODIMENTS OF THE INVENTION

The following are illustrative examples of the invention in which allparts and percentages are by weight and all degrees are Celsius unlessotherwise stated.

EXAMPLE 1 Anionic Polymerization of Formaldehyde ##STR14## Lithiumo-Nitrobenzoxide Initiator

A resin kettle polymerization reactor was fitted with a gas inlet tube,a T tube attached to a bubbler and a dry argon source, and a vibromixerstirrer. The apparatus was flame-dried and cooled under a constant flowof dry argon. Into this reactor was placed 250 ml of freshly distilledtetrahydrofuran (THF), previously dried over sodium, and 1.52 g (0.010mole) of recrystallized o-nitrobenzyl alochol. To this mixture was thenadded 2.5 ml of 1.6 molar (0.004 mole) n-butyllithium in hexane to forman equivalent amount of lithium o-nitrobenzoxide. The excesso-nitrobenzyl alcohol was present to serve as a chain transfer agent. Ina second flame-dried resin kettle formaldehyde generator was placed 200ml of decahydronaphthalene, 50 g of paraformaldehyde and 5 g of succinicanhydride. Succinic anhydride served as a scavenger for water to preventits passage into the reactor where it could function as a chain-transferagent. This generator was attached to the reactor via two traps whichwere cooled to -10° by ice-methanol baths. A stream of argon carrier gaswas passed through the generator solution. The generator was then heatedto 125°-150° (oil bath temperature set at 180°) and the formaldehydepassed through the traps into the reactor via the gas inlet tube whichwas inserted below the THF surface. White crystalline polymer formedcontinuously during the reaction period of approximately one hour.Completion of the reaction was indicated by a clearing of thedecahydronaphthalene solution and the continued increase of thegenerator pot temperature to about 150°. The reaction was terminated bythe addition of 5 ml of acetic acid, and the intermediate insolublecrystalline polymer was collected by filtration, washed with THF,refiltered and air dried.

The above intermediate polymer was stablilized by capping the terminalhydroxyl ends by reaction with acetic anhydride in the following way.The polymer was placed in a round-bottomed flask fitted with a refluxcondenser, thermometer and magnetic stirrer. To the flask was added 100ml of acetic anhydride and 1.0 g of anhydrous sodium acetate. Themixture was heated rapidly to reflux at 140° and held at thistemperature for 45 minutes. The polyoxymethylene polymer dissolved inthe refluxing acetic anhydride. On cooling to room temperature, polymerprecipitated as a very finely divided powder. The polymer was recoveredby filtration, washed extensively with methanol, refiltered, and vacuumdried to give an overall yield based on paraformalehyde of 14% (7.16 g).The onset of major melting of the polymer was 137°-140°, and thecalculated average degree of polymerization (DP) of the polymer chainswas approximately 25. An infrared spectrum indicated that nitro groupshad been incorporated into the polymer with about 65% of the chainsinitiated by o-notrobenzoxy groups.

A sample of the polymer was irradiated in a Pyrex® test tube by rollingthe tube under a 275 watt sunlamp at a distance of 12 inches for periodsof 30 minutes and 60 minutes. These samples were compared by DTAanalysis to both unirradiated samples and unstabilized samples. Theunirradiated reference polymer started melting about 140° with themaximum rate at 153°. No polymer weight loss occurred through threeheat-cooling cycles through the melting point to 175°. The unstabilized(uncapped) polymer totally degraded on the first heating with thedegradation onset ocurring at 95°-119°, similar to the result obtainedfor uncapped paraformaldehyde. The irradiated samples showed degradationand weight loss occurring starting at 98°-119° with some melting ofunactivated polymer at 140° to 153°. Subsequent heating cycles showedmelting of still unactivated polymer at 140° to 153°C.

EXAMPLE 2 Anionic Polymerization fo Formaldehyde with Lithiumo-Nitrobenzoxide Initiator

The polymerization described in Example 1 was scaled up as follows. Theformaldehyde generator was charged with 200 g of paraformaldehyde, 400mlof decahydronaphthalene and 40 g of succinic anhydride. Thepolymerization reactor was charged with 500 ml of freshly distilled THFand 5.1 g (0.033 mole) of o-nitrobenzyl alcohol. This reactor was cooledto -30 to -50° with an ice-methanol-dry ice cold bath. To the reactorwas then added 10 ml (0.016 mole) 1.6 n n-butyllithium in hexane. Thegenerator was heated to 150° during which time a constant stream ofargon carrier gas was passed through the mixture transporting theformaldehyde through the cold traps and into the reactor. Insolublepolymer started forming immediately and continued to do so until all ofthe paraformaldehyde had been decomposed. The reaction was terminated byadding 10 ml of acetic acid, and the product was filtered, washed withadditional THF, and refiltered.

The above intermediate polymer was encapped as described in Example 1with 500 ml of acetic anhydride and 1.0 g of anhydrous sodium acetate at140° for 30 minutes. The capped polymer was washed and dried, asdescribed, and a yield of 65 g (33%) of polymer was obtained. Infraredanalysis showed that 68% of the polymer chains were initiated with ano-nitrobenzoxy group. The onset of major melting of the polymer was140°, and the calculated average DP was about 30.

A sample of the polymer was irradiated for one hour as described inExample 1. DTA analysis of the irradiated sample indicated onset ofdegradation at 100° resulting in approximately 50% weight loss andconsiderable discoloration of the sample, apparently caused by reactionproducts derived from liberated o-nitroso-containing end groups. Anunirradiated polymer sample showed no degradation upon heating andcooling for three cycles from 25°-175°, and no discoloration of thesample occurred.

CONTROL EXAMPLE A Anionic Polymerization of Formaldehyde withp-Nitro-benzoxide as Initiator

This experiment demonstrated that a polyoxymethylene polymer capped witha p-nitrobenzoxy end group did not undergo photodecomposition.

The polymerization was carried out using the reactants and proceduredescribed in Example 2 with the exception that o-nitrobenzyl alcohol wasreplaced with 5.1 g of p-nitrobenzyl alcohol. From 200 g ofparaformaldehyde, the yield of acetate-capped polymer was 28.5 g (14%).Infrared analysis showed that 70% of the polymer chains were capped withp-nitrobenzoxy groups. The molecular weight, calculated from theobserved melting point onset of 152°, corresponded to a DP ˜ 33.

A sample of the polymer was irradiated for 1.5 hours as described inExample 1. DTA analysis of the irradiated polymer was identical withthat of an unirradiated control sample; no degradation was observedthrough three heat-cool cycles to 180° .

EXAMPLE 3 Anionic Polymerization of Formaldehyde with sodiumo-Nitrobenzoxide as Initiator

The polymerization was carried out using the reactants and proceduredescribed in Example 2 with the exception that the n-butyllithiumsolution was replaced with 0.8 g of a 50% dispersion of sodium hydridein mineral oil (0.016 mole sodium hydride). When reaction between thesodium hydride and o-nitrobenzyl alcohol was complete (hydrogenevolution ceased), formaldehyde was generated and passed into thereactor in the usual way. The yield of product, after endcapping withacetic anhydride, amounted to 28.38 g (14%). Infrared analysis showedthat essentially all of the polymer chains were capped witho-nitrobenzoxy groups, and melting point data showed a DP ofapproximately 30; onset of major melting, 140°.

A sample of powdered polymer was irradiated for 1 hour with a 275 wsunlamp placed 12 inches from an open dish which contained the polymersample. Total degradation (indicated by DTA analysis) of this sampleoccurred; degradation endotherm ˜ 98°. An unirradiated reference sampledid not show any degradation upon three heat-cool cycles to 180°.

EXAMPLE 4 Anionic Polymerization of Formaldehyde with Lithium (TMEDA)₂o-Nitrobenzoxide Initiator

The polymerization was carried out using the procedure described inExample 2. The polymerization reactor was charged with 500 ml of freshlydistilled sodium-dried cyclohexane and 5.0 grams of o-nitrobenzylalcohol. To this mixture was added 0.4 ml (0.0033 mole) of freshlydistilled N,N,N',N'-tetramethylethylenediamine (TMEDA) followed by 10 mlof 1.6 N butyllithium in hexane. The reaction mixture was cooled with anice-water bath but not sufficiently to freeze the cyclohexane.Formaldehyde was then passed into the reactor in the usual way (200 g ofparaformaldehyde was employed in the formaldehyde generator). Thereaction was terminated by the addition of 20 ml of acetic acid when theparaformaldehyde was completely decomposed, and the polymer wasseparated by filtration. The polymer was endcapped as described inExample 2 to give 57 g of capped polymer (28.5% yield). The onset of thepolymer melting point was 139°-140° corresponding to a DP of about 30.Infrared analysis showed that 79 % of the polymer chains were cappedwith o-nitrobenzoxy groups.

A sample of the powdered polymer was irradiated as described in Example3. Thermal decomposition of the irradiated polymer (DTA) occurred at 96°whereas nonirradiated polymer was stable through three heat-cool cyclesto 175°.

EXAMPLE 5 Anionic Polymerization of Formaldehyde with Tetrabutylammoniumo-Nitrobenzoxide as Initiator

The polymerization was carried out using the procedure and apparatusdescribed in Example 2. The polymerization reactor was charged with 500ml of hexane, 5.0 g of o-nitrobenzyl alcohol and 10 ml (0.016 mole) of1.6 N n-butyllithium in hexane. When the initial reaction had stopped,4.43 g (0.016 mole) of tetrabutylammonium chloride was added.Formaldehyde was passed into the reactor in the usual way (200 g ofparaformaldehyde was employed in the formaldehyde generator), and thepolymerization reactor was maintained at about 25° by cooling with icewater. When the paraformaldehyde was completely decomposed,polymerization was terminated by the addition of 25 ml of an aceticacid-acetone mixture. The resulting polymer was separated by filtration,dried (34.2 g) and endcapped with acetic anhydride (400 ml) as describedin Example 2 to give 25.8 g (12.9% yield) of capped polymer. Infraredanalysis showed that 64% of the polymer chains were capped witho-nitrobenzoxy groups; DTA melting range = 137°-140° corresponding to aDP of 28-30.

A sample of the powdered polymer was irradiated as described in Example3. Thermal decomposition of the irradiated polymer (DTA) occurred at100° whereas nonirradiated polymer was stable through three heat-coolcycles to 175°.

EXAMPLE 6 Anionic Polymerization of n-Butyraldehyde with Lithium(TMEDA)₂ o-Nitrobenzoxide as Initiator

A resin kettle polymerization reactor was fitted with an inlet port, a Ttube attached to a bubbler and a source of dry argon, and a vibromixerstirrer. The apparatus was flame-dried and cooled under a constant flowof dry argon. About 200 ml of 30°-60° petroleum ether (previously driedover sodium) was distilled into the reactor and 100 ml of freshlydistilled n-butyraldehyde was added.

The initiator solution was prepared in a separate dry, nitrogen-filledbottle. It consisted of 50 ml of THF (dried over sodium), 3.0 g ofo-nitrobenzyl alcohol and 10 ml of 1.6 N n-butyllithium in hexane. Thebutyllithium solution was added slowly while the bottle was vented withnitrogen. After the reaction was completed, 2.0 ml of TMEDA was added toassist in solubilizing the initiator in the polymerization reactionmedium. The initiator solution was added all at once to thepolymerization reaction kettle, cooled to -78°. The reaction mixturegradually became more opaque and viscous. After 3 hours polymerizationwas terminated by the addition of a solution of 15 ml of acetic acid in100 ml of ethanol while the temperature was maintained at <-40°. Thecrude polymer was separated by filtration, washed with ethanol, anddried. It was endcapped by reaction with a solution of 200 ml of aceticanhydride and 60 ml of pyridine at reflux temperature for 0.5 hour. Thesolution was cooled and the precipitated polymer was separated, washedwith acetone, and dried to give 7.04 g (8.6% yield) of acetate-cappedpoly-n-butyraldehyde. Infrared analysis showed that essentially all ofthe polymer chains were capped with o-nitrobenzoxy groups.

A sample of the polymer was irradiated for 0.5 hour with a 275 w sunlampplaced 12 inches from an open dish which contained the polymer sample.Irradiation resulted in formation of color in the sample and a slightshift in the infrared absorption hand of the nitro group. Thermaldepolymerization of the irradiated polymer (DTA) occurred with evolutionof butyraldehyde starting at 77°. The unirradiated sample was stable to200° with no evidence of melting or decomposition.

EXAMPLE 7 Anionic Polymerization of Formaldehyde with the LithiumAlkoxide of Benzoin as Initiator

The polymerization was carried out using the procedure and apparatusdescribed in Example 2. The polymerization reactor was charged with 250ml of freshly distilled THF (sodium dried) and 2.2 g of benzoin. Theformaldehyde generator was charged with 50 g of paraformaldehyde and 200ml of decahydronaphthalene. The generator was heated to 150°, and whenthe evolution of formaldehyde began, 2.5 ml of 1.6 N n-butyllithium inhexane was added to the cooled (-30° to -50°) polymerization reactor.The reaction mixture initially turned yellow and then faded to almostwhite as the insoluble polyformaldehyde formed. When all of theparaformaldehyde had been decomposed, 5.0 ml of acetic acid was added tothe reactor to terminate the reaction. The polymer was isolated byfiltration, washed with additional THF, filtered, and air dried (21.8 g;43.6% yield). It was endcapped as described in Example 2 (200 ml ofacetic anhydride). Infrared analysis of the acetate-capped polymershowed the presence of both acetate (5.78μ ) and benzoyl carbonyl (5.9μ)absorption bands. Comparison of the ratio of these two bonds with areference benzoin ether/amyl acetate mixture indicated 57% of the chainswere capped with benzoin ether units. DTA melting point data forunirradiated polymer indicated a double break at 109° (DP = 19) and 137°(DP˜30).

Samples of the benzoin-containing polymer and the polymers of Example 1were irradiated in open aluminum pans at a distance of 25 inches fromthe sunlamp for 1.5 hours. Samples were removed at 15-minute intervals,weighed, and analyzed by DTA. Thermal degradation of the irradiatedsamples occurred compared with the unirradiated controls. However, therate of weight loss for the o-nitrobenzyl-capped polymer wasapproximately 11 times that of the benzoin-capped polymer.

When a sample of the benzoin-capped polymer was irradiated in a testtube under an argon atmosphere for 1.5 hours by the procedure of Example1, it did not show significant thermal degradation compared with anunirradiated control sample.

EXAMPLE 8 Anionic Polymerization of Formaldehyde with Lithiumo-Nitrobenzoxide Initiator

The procedure and reactants used were those of Example 1 with thefollowing modifications. The polymerization reactor was cooled at -30°during the polymerization, and the butyllithium solution was added atthe onset of formaldehyde generation. The reaction was terminated by theaddition of acetic acid when the inlet tube became plugged with polymer.

The crude polymer (14.45 g) was endcapped by reaction with 100 ml ofacetic anhydride and 25 ml of pyridine at 125°. The recovered, driedpolymer weighed 10.96 g. Infrared analysis showed that the majority ofthe polymer chains were endcapped with o-nitrobenzoxy groups; DTAmelting point = 141° corresponding to an average DP˜30.

Irradiation of the polymer as described in Example 1, showedphotosensitivity resulting in polymer degradation upon heating (DTAanalysis) after 30 minutes and 60 minutes exposure times.

EXAMPLE 9 Endcapping of Polyformaldehyde with o-Nitrobenzyl Groups

A 500 ml 3-necked flask was equipped with a mechanical stirrer,thermometer and nitrogen bubbler. Fifty grams of uncappedpolyoxymethylene polymer of molecular weight ˜130,000 was added, thenitrogen bubbler was replaced with a vacuum line, and the polymer wasdried at 160°, 0.5 mm pressure, for 60 minutes with stirring. Nitrogenwas added, the flask was cooled to 40°-50°, and 150 ml of heptane, driedby distillation from calcium hydride, and 4.8 g of tris(o-nitrobenzyl)orthoformate were added. The reaction mixture was heated with stirringat 73° for 15 minutes, 0.2 ml of boron trifluoride etherate was added,and the reaction mixture was stirred at 73°-75° for one hour. It wascooled to room temperature, 5 ml of tributylamine was added, and themixture was stirred for 15 minutes. The capped polymer was separated byfiltration, washed successively with methanol and with acetone, andfinally dried at 60° (0.2 mm), yield 40.08 g, mp (capillary tube),157°-160°.

A film of this polymer, first cold pressed and then pressed at 170°, wasexposed to radiation from a mercury resonance lamp for 10 minutesthrough a process transparency. The exposed film was thermally developedby heating it in a circulating air oven at 160° for 0.5 hr. Imaging wasobserved, although some degradation of the unexposed areas of the filmwas noted.

To improve its thermal and base stability, a 5.0 g sample of the polymerwas post-treated by suspension in a mixture of 30 ml of benzyl alcoholand 1.5 ml of tributylamine followed by a 30-minute nitrogen purge. Thepolymer was dissolved by heating the suspension in an oil bath preheatedto 165°, and the solution was further heated at ˜140° for one hourfollowed by cooling under nitrogen. The precipitated polymer was washedwith acetone and dried, yield 0.99 g (20% recovery). Ultravioletabsorption analysis in hexafluoroisopropanol solvent showed the presenceof 156 formaldehyde units/o-nitrobenzyl chromophoric group. Theultraviolet absorption band at 267 nm was employed for this analysis,and a molar extinction coefficient, ε = 4500, was used as a referencefor o-nitrobenzyl groups, obtained from the value foro-nitrophenylethylene glycol.

The experiment was repeated as described, and the polymer recovery aftertreatment with benzyl alcohol-tributylamine was 0.64 g (12.8%), mp156°-158°. A film of this polymer was exposed to radiation, asdescribed, to give an image with good relief. In these instances apositive image was obtained, i.e., polymer remained under the opaqueareas of the process transparency, that is the areas not struck byradiation passing through the transparency.

EXAMPLE 10

This example demonstrates the superior photothermal decompositionproperties of the polymers of this invention compared with theintercalated polymers of the common assignee's U.S. Pat. No. 3,991,033.

TEST PROCEDURE A

Samples of each polymer were melt-pressed into thin films on weighedaluminum plates, each sample having an oriented polyester coversheet.The coversheets were removed and the weight of each sample determined.The samples were then exposed simultaneously to a 275 w sunlamp placedtwo feet from the plates for a period of 1 hour. The positions of theplates were rotated every 15 minutes so that each sample was in eachlocation for an identical period of time. Following exposure each platewas again weighed and weight loss accompanying exposure determined. Itwas found that no sample changed significantly during exposure. Thesamples were then heated on a regulated hot plate at 180° for 10minutes. The appearance of each sample was recorded and the weight lossdetermined.

TEST PROCEDURE B

Samples were melt pressed onto preweighed aluminum sheets. Afterobtaining their weights, the samples were in turn heated with melting at180° C for 10 minutes on a regulated hot plate. They were then cooledand their weight loss determined. No significant loss occurred for anysample. The samples were again heated to 180° C and the molten polymerswere irradiated with a 250 w sunlamp set at a distance of eight inchesfor 15 minutes. Finally the samples were removed, cooled to roomtemperature and physical appearance and weight loss were measured.

The results from these experiments are summarized in Table I.

The intercalated polyoxymethylene control polymer was prepared asfollows. Uncapped polyoxymethylene (110.08 g MW˜60,000) was dried invacuo in a 500 ml 2-necked flask at 60°-120° for 4.5 hours. The centerneck had a stopcock adaptor to a vacuum pump, and the side arm wasfitted with a serum cap. The weight of dried polymer was 100.49 g (8.7%loss). A nitrogen bubbler was substituted for the vacuum pump and 240 mlof heptane (freshly distilled from calcium hydride) and 30 ml of freshlydistilled 4-o-nitrophenyl-1,3-dioxolane were added via syringe throughthe serum cap under nitrogen. The nitrogen bubbler valve was closed, andthe reaction mixture was immersed in a preheated 75° bath for 40 minuteswith stirring. After the addition of 0.05 ml of freshly distilled BF₃·(C₂ H₅)₂ O the slurry was maintained at 70°-80° for one hour. Thereaction was quenched with 10 ml of tributylamine, and the product wascooled, filtered, and washed thoroughly with methanol and acetone. Theweight of dried, light sensitive, intercalated polymer, obtained as acolorless or very pale yellow solid, was 99.6 g.

To improve its thermal and base stability, the polymer was post-treatedby suspension in 600 ml of benzyl alcohol and 30 ml of tributylaminefollowed by a 2.2 hour nitrogen purge. The polymer was dissolved byheating the suspension to 160° over a period of 25-30 minutes, and theclear yellow solution was further heated at 160° for 15 minutes followedby rapid cooling with an ice bath. The swelled and voluminousreprecipitated polymer was filtered, washed extensively with methanoland acetone, and dried in vacuo at 70°. The weight of purified polymerwas 45.5 g. The purified polymer was combined with that obtained fromtwo similar intercalation experiments (116.2 g total), and the mixtureretreated with 600 ml of benzyl alcohol and 30 ml of tributylamine asdescribed. The dried product amounted to 115.1 g (> 99% recovery), η inh= 0.63 (30°, 0.5% in hexafluoroisopropanol (HFIP), MW ˜ 15,000).Ultraviolet analysis (HFIP) showed the presence of 178-128 formaldehydeunits/oxyethylene unit.

The concentration of oxyethylene groups was determined by ultravioletspectroscopy using the molar extinction coefficient of the aromaticsubstituent which was obtained from the 4-o-nitrophenyl-1,3-dioxolanestarting material. A band at 267 nm was employed having an extinctioncoefficient ε = 4,500.

                                      Table I                                     __________________________________________________________________________    Comparative Photothermal Sensitivity of Photosensitive Polyoxymethylenes                                    Relative                                        Sample     Test               Reaction                                        No.  Sample                                                                              Procedure                                                                           Corrected % of wt. Loss*                                                                   Rate Remarks                                    __________________________________________________________________________    1    Example 3                                                                           A     36           1.91 Discolored on irradiation;                                                    deep amber-brown residue after                                                heating; evolution of formal-                                                 dehyde gas                                 2    Example 2                                                                           A     36.8         1.96 Same as Sample 1                           3    Example 4                                                                           A     25.2         1.34 Same as Sample 1                           4    Control                                                                             A     18.8         1    Less discolored than Samples 1,                 Polymer                       2, 3 after exposure; fractured,                                               brittle, discolored polymer after                                             heating                                    5    Example 3                                                                           B     70           1.37 Light beige melt on heating;                                                  dark oil formed during                                                        irradiation accompanying                                                      evolution of formaldehyde gas;                                                deeply discolored residue                                                     after exposure                             6    Example 2                                                                           B     79           1.54 Same as Sample 5                           7    Control                                                                             B     51           1    Light beige melt on heating; some               Polymer                       darkness on exposure with evolu-                                              tion of formaldehyde; brittle                                                 polymeric residue after                    __________________________________________________________________________                                       exposure.                                   *Corrected weight loss = % weight loss of volatile component of polymer  

EXAMPLE 11

This example demonstrates the utility of photothermosensitivepolyaldehydes as a source of aldehyde, useful for crosslinkingpolyamides, which have photoimaging, adhesive, and coating applications.

A mixture of 5.0 g of Elvamide® Nylon Resin (8023), 1.0 g of thepolyoxymethylene of Example 8 and 50 ml of methanol was heated to 60°with stirring to dissolve the polyamide resin, and this dispersion wassprayed onto a shiny aluminum panel. The panel was cooled and dried. Itwas exposed for 5 minutes in a vacuum frame through a processtransparency. The radiation source was a 275 watt sunlamp mounted 10inches from the coated panel. After exposure the panel was developedthermally by heating at 150° for 15 minutes in a circulating air oven. Agood image with good relief was obtained in the radiation-struck areasafter removal of the uncrosslinked nylon resin with warm methanol. Somelarge polyoxymethylene particles which were poorly dispersed werevisible.

B

Example 11-A was repeated with Elvamide® 8061 Nylon Resin replacing the8023 resin. Imaging was again obtained, but relatively poor adhesion tothe aluminum plate was noted in the absence of a conventional adhesionpromoter.

C

A mixture was prepared by dissolving 2.5 g Elvamide® 8061 Nylon Resinand 0.5 g Ciba® 825 polyaminepolyamide resin in 25 ml of methanol. In asecond container 1.0 g of the polyoxymethylene of Example 8 was finelydispersed in 25 ml of butanol. The two mixtures were combined and thenheated to drive off the methanol leaving all polymeric materialssuspended in the residual butanol. This mixture was then sprayed ontoshiny aluminum panels and allowed to dry.

Samples were irradiated as described in Example 11-A above for 5 minutesand then thermally treated in a circulating air oven at 150° for periodsof 1, 2, 3, 4, 5, 7, and 10 minutes. The images were finally developedby washing with warm methanol. Adhesion of imaged material to thesubstrate was excellent. Good relief images were obtained for allsamples heated for 1-7 minutes. Some background developed for samplesheated for 7 and 10 minutes.

What is claimed:
 1. A photosensitive polyaldehyde having the formula:##STR15## wherein R is a photosensitive end group selected from##STR16## R¹ is H or n-alkyl of 1-5 carbon atoms, R² is (a) n-alkanoylof 1-4 carbon atoms or (b) n-alkanoyl of 1-4 carbon atoms or ##STR17##when R¹ is H, and n is a positive integer of about 10-4000.
 2. Thepolyaldehyde of claim 1 wherein R¹ is H.
 3. The polyaldehyde of claim 2wherein R is ##STR18##
 4. The polyaldehyde of claim 2 wherein R is##STR19##
 5. The polyaldehyde of claim 3 wherein R² is acetyl and n isabout 15-300.
 6. The polyaldehyde of claim 4 wherein R² is acetyl and nis about 15-300.